Electron gun structure for producing an electron beam free of radial velocity components wherein the length of the first non-magnetic cylinder is approximately equal to an integral number of wave lengths of the scallop frequency



Nov. 2, 1965 A. s. BLUM 3,215,890

ELECTRON GUN-STRUCTURE FOR PRODUCING AN ELECTRON BEAM FREE OF RADIALVELOCITY COMPONENTS WHEREIN THE LENGTH OF THE FIRST NON-MAGNETICCYLINDER IS APPROXIMATELY EQUAL TO AN INTEGRAL NUMBER OF WAVE LENGTHS OFTHE SCALLOP FREQUENCY Filed May 22, 1961 Ff'a. 1

lNVE/VTOR Ash r 5 15211221 %MMM ,4 TOR/V5 Y United States Patent3,215,890 ELECTRON GUN STRUCTURE FOR PRODUCING AN ELECTRON BEAM FREE OFRADIAL VE- LGCITY COMPONENTS WHEREIN THE LENGTH OF THE FIRSTNON-MAGNETIC CYLINDER IS APPROXIMATELY EQUAL TO AN INTEGRAL NUMBER OFWAVE LENGTHS ()F THE SCAL- LOP FREQUENCY Asher S. Blunt, Chicago, Ill.,assignor to Zenith Radio Corporation, a corporation of Delaware FiledMay 22, 1961, Ser. No. 111,831 7 Claims. (Cl. 315-31) The presentinvention is directed to an electron gun arrangement of particular valuefor low voltage electron discharge devices such as the electron beamtransversemode parametric amplifier.

The gun structure is an improvement over that described and claimed incopending application Serial No. 822,267, filed June 23, 1959, in thename of George W. Hrbek, and assigned to the same assignee as thepresent invention, now Patent 2,983,842, issued May 9, 1961.

An electron gun of the type under consideration is utilized to produceand project, along a path which extends coaxially of a homogeneousunidirectional magnetic field, an electron beam having the optimumcondition of substantially no radial component of motion. That is tosay, the gun is an arrangement for achieving in a practical manner abeam in the form of a solid cylindrical rod projected coaxially of ahomogeneous magnetic focusing field while concurrently rotating aboutits longitudinal axis. This optimum beam condition is usually referredto as a condition of balance attained between the space charge andcentrifugal forces acting on the electrons on the one hand and thefocusing force of the magnetic field on the other.

If achieved, it permits the transverse-mode parametric amplifier whichis inherently a low noise device to exhibit an optimum noise figurebecause it represents a condition of minimum beam noise. The practicalsignificance is apparent when it is recognized that a slight mismatchmay occur in the input coupler of the amplifier through which signalenergy is transferred to the beam for amplification and any suchmismatch reflects noise back into the electron stream to adverselyaffect the noise figure.

The gun structure of the Hrbek application has proved to be anacceptable and satisfactory approach to the problem of producing thedesired electron flow for the beam of a transverse-mode parametricamplifier. It has been discovered, however, that the noise figure ofsuch an electron gun has an undesirable dependence upon .the potentialsof its several electrodes. The electron gun of the present invention isan improvement over the Hrbek structure in that it materially reducesthe dependence of noise figure upon gun potentials which facilitatesadjustment of the amplifier. It also reduces to a minimum the number ofelectrodes required to accomplish optimum conditions of flow of theelectron beam.

In principle, the desired objective is accomplished by a gun structurecomprising a cathode immersed in a magnetic field and an adjacent anodehaving an aperture which determines the initial beam diameter. As thebeam emerges from the anode aperture it must diverge due to the lenseffect of the anode and to existing excessive space charge forces andthis perturbation of the beam gives rise to a sinusoidal component ofradial motion. The

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perturbed beam is permitted to drift for awhile and at the right timethe perturbation is stopped so that the beam exhibits substantially nocomponent of radial motion.

It is therefore a principal object of the invention to provide a new andimproved electrode system for producing a confined flow electron beamwhich has no substantial component of radial motion.

Another principal object of the invention is to provide an electron gunfor a beam type parametric amplifier in which the noise figure is lessdependent on operating potentials of the electrodes and in which the gunelements are reduced in number to simplify both the gun structure andits power supply.

It is another particular object of the invention to provide a novelelectron gun system for a parametric amplifier which permits theamplifier to exhibit improved noise properties.

An electrode system or electron gun, embodying the present invention,produces and projects an electron beam along a path which extendscoaxially of a homogeneous unidirectional magnetic field. The guncomprises a cathode and an aperture anode. Usually, the anode apertureis small in area compared with the cathode and it is centered on thedesired beam path. Means are provided for operating the anode at apositive potential with respect to the cathode to draw electronstherefrom into a beam having an initial radius determined by the anodeaperture. Such a beam, as it emerges from the anode aperture, expandsradially outwardly and this perturbation gives rise to a sinusoidalcomponent of radial motion of a predetermined scallop frequency. The gunhas other means, including a cylinder of non-magnetic conductivematerial positioned coaxially of the beam path, for establishing aboutthat path a substantially electric fieldfree drift space for theperturbed beam. The cylinder is dimensioned so that the length of thedrift space is approximately equal to an integral number of wavelengthsat the scallop frequency. Finally, there are means for terminating thedrift space in a convergent lens which has a principal plane located atthe intercept of the perturbed beam with an equilibrium radius largerthan the initial beam radius and which subjects the perturbed beam to aradially inward force to cancel its component of radial motion.

The features of the present invention which are believed to be novel areset forth with particularity in the appended claims. The organizationand manner of operation of the invention, together with further objectsand advantages thereof, may best be understood by reference to thefollowing description taken in connection with the accompanyingdrawings, in the several figures of which like reference numeralsidentify like elements, and in which:

FIGURE 1 is an enlarged and schematic representation of an electron gunsystem constructed in accordance with the invention; and

FIGURE 2 represents a modification of the structure of FIGURE 1.

Referring now more particularly to FIGURE 1, the dash-dot constructionline 1010 designates a path along which an electron beam is to beprojected. In most cases this path is the central axis of the tubewhich, as represented in the drawing, has an envelope 11 enclosing theelectron gun and the other structures constituting a transverse-modetype of parametic amplifier. Since the invention concerns mostparticularly the electr-on gun as distinguished from the othercomponents of the amplifier, the illustration and the text are confinedlargely to the gun and reference is made to the above-identified Hrbekapplication for a representation of the remaining structural componentsof such an amplifier. In addition to the electron gun, such an amplifiernormally includes, in the recited order, an input coupler, a modulationexpander, an output coupled and a collector electrode. Of theseelements, only input coupler 12 and collector electrode 13 have beenillustrated and the break shown in the representation of envelope 11 aswell as the beam path are intended to indicate the om1ss1on.

The parametic amplifier also has a homogeneous unidirectional magneticfield represented in FIGURE 1 by arrow H. This is a focusing field whichis usually established by means of a solenoid arranged coaxially of thebeam path and coupled to a direct current source of adjustable magnitudeto facilitate the development of a focusing field of a desired value.Again, the structure of this solenoid has been omitted in order tosimplify the drawing. I V

The overall operation of such an amplifier is Well understood and neednot be developed in detail. Suffice it to say that a signal to beamplified is applied to input coupler 12 to deflection-modulate theelectron beam after which the beam enters the modulation expander wherethe modulation, represented'by transverse electron motion, is expanded.Thereafter, the beam with its expanded modulation traverses an outputcoupler where the amplified signal energy is taken off and applied to aload. The remainder of this description will be confined to thedevelopment of the electron beam which, as presented to the field ofinput coupler 12, has no substantial radial component of motion.

The electron gun is of the immersed type, comprising a cylindricalcathode 15 immersed in magnetic field H and having an end cap 16 coatedwith electron emissive material. As is usually the case, the cathode isindirectly heated by a filament enclosed within cylinder 15 as indicatedby the fragmentary section 17. A thin planar anode 20 is disposed acrossbeam path 10 and spaced axially a short distance from emitting surface16 of the cathode. The anode is provided with a centrally locatedaperture 21 which is small in area compared with emitting surface 16 ofthe cathode and is centered on the beam path.

The application of a positive potential to anode 20, through meanspresently to be described, causes current flow between the cathode andthe anode as shown by the broken-construction lines extendingtherebetween. This current will be spaced charged limited and theelectrons drawn through anode aperture 21 constitute the useful electronbeam which has an initial radius determined by and correspondingessentially to the anode aperture. As the beam emerges from the anodeaperture, its expands radially outwardly and this perturbation givesrise to a sinusoidal component of radial motion of a predeterminedscallop frequency, as indicated by the sinusoidal beam envelope 22. Thescallop frequency, if space charge effects are ignored, is equal to thecyclotron frequency of the amplifier which may be adjusted by adjustmentof focusing field H. When space charge effects are taken intoconsideration, however, the wavelength corresponding to the scallopfrequency is slightly larger, perhaps 10% larger, than the cyclotronwave length. It may be shown that the scallop wavelength is determinedby the beam current, the size of anode aperture 21 and the beamvelocity, assuming a fixed value for the axial magnetic field. Thesinusoidal perturbation of the beam is a manifestation of a radialcomponent of motion which is undesired and which the structure underconsideration effectively suppresses. The construction line r is theequilibrium radius of the beam and is the radius at which the opposingradially directed forces acting upon the electrons are equal.

Next along the beam path, following anode 20, are means for establishingabout the path a substantially electric-field-free drift space for theperturbed beam. This means in the modification of FIGURE 1 comprises afirst cylindrical electrode 25 of non-magnetic conductive ma- .terialsupported coaxially of the beam path. The cylinder has a diameter onlyslightly larger than the beam diameter and may have a centrallyapertured end plate at the end thereof closer to cathode 15 which mayconveniently be employed as anode 20. The other end 26 of the cylindermay have an inwardly directed flange, as shown, to serve as an elementof a lens to be considered presently.

Positioned next adjacent cylindrical electrode 25 in the direction ofcollector 13 is a second cylindrical electrode 30 which constitutesmeans for terminating the drift space provided for the perturbed beam ina convergent lens. This electrode is quite similar to electrode 25,being constructed of non-magnetic conductive material and positionedcoaxially of the beam path. Its end portions 31 and 32 having inwardlydirected flanges and its terminal portion 31 serves as a lens element,cooperating with end portion 26 of cylinder 25, to constitute aconvergent lens. Additionally, cylinder 30 establishes a second driftspace for the beam which leads directly to input coupler 12.

It is not necessary to use flanged terminations at end portions 26, 31and 32 of cylinders 25 and 30 if the diameters of these cylinders havetheir preferred dimensions which is only slightly larger than themaximum beam diameter.

The principal plane of the convergent lens provided by electrodesections 26 and 31 is represented by broken construction line 33 and theeffective electrical length of the drift space established bycylindrical electrode 25 is chosen to be approximately equal to anintegral number of wavelengths at the scallop frequency. For many gunstructures the length of this drift space falls between n and (mi- A)wavelengths at the scallop frequency, where n is any integer includingone. The dimensioning of the drift space is such that the lens plane 33is located at the intercept of the perturbed beam with an equilibriumradius r within cylinder 30. This equilibrium radius is always greaterthan the initial beam radius determined by anode aperture 21 and isgreater than equilibrium radius r within cylinder 25 for any gunstructure in which the operating potential of cylinder 30 is less thanthat of cylinder 25. Since equilibrium radius r exceeds the initial beamradius, its intercept with the perturbed beam occurs at a distance fromanode aperture 21 which is greater than a scallop wavelength.

In addition to locating the lens plane appropriately as described, it isnecessary that the strength of the lens be adjusted by selection of therelative potentials of lens elements 26 and 31 to subject the perturbedbeam to a radially inward force which cancels the component of radialmotion developed by the beam through its expansion as it emerges fromanode aperture 21. Accordingly, cylinders 25 and 30 have connectionsdesignated V and V respectively, leading to a power supply (not shown)from which operating potentials are applied to the cylindricalelectrodes. Cylinder 25 is positive with respect to the cathode andelectrode 30 is usually less positive than electrode 25 in an amount toestablish the desired strength of convergent lens 26, 31. As aconsequence of the compensating effect of the convergent lens, theperturbation of the beam is wiped out and the beam traverses the driftspace of cylinder 30 and enters input coupler 12 with no substantialcomponent of radial motion.

In selecting operating potentials V and V it is especially beneficial tohave potential V approximately equal to the direct current operatingpotential of input coupler 12. This avoids undesired lens effects thatmay otherwise arise if cylinder 30 and input coupler 12 are atsubstantially different potential levels.

The modification of FIGURE 2 features sectionalizing of cylindricalelectrodes 25 and 30. More particularly, electrode 25 is here formed ofthree mutually insulated cylindrical sections 25a, 25b and 250, theinsulation therebetween being designated 25d. This modification alsoindicates that anode 20 may be a separate, thin plate conductivelyafiixed with the adjacent end of cylindrical section 25a to increaseheat dissipation at the anode. Of course, as is clear from thediscussion of FIGURE 1, it is not essential that a separate anodeelectrode be employed. Providing the individual cylindrical electrodesections 25a-c with terminals V to V respectively, increases theflexibility of the system by permitting the cylindrical sections tooperate at approximately the same potentials but to obtain optimumperformance through fine adjustment.

The second cylinder is likewise divided into sections shown as 30a and30b, insulated from one another by in sulation 30d. These sections alsohave separate terminals V and V respectively, to permit fine adjustmentof the nominally equal values of operating potential. Between cylinder30 and input coupler 12 is an end wall 35 of a cylindrical shieldhousing which may enclose the coupler.

Obviously, it electrode sections 25a-c operate at the same potential,segmented cylindrical electrode 25 of FIG- URE 2 is electrically thesame as the corresponding electrode 25 of FIGURE 1. Similarly, operatingsections 30a and 30b of cylinder 30 at a common potential causes theembodiment of FIGURE 2 to be electrically identical to that of FIGURE 1but, the provision of separate terminals permits more flexibility ofadjustment than is possible With the first described arrangement.

For example, variation of the potential applied to terminal V permitsadjustment of the effective electrical length of cylinder 25 properly tolocate the principal plane of the convergent lens defined by electrodes25c and 30a to cancel out the perturbation developed on the beam atanode aperture 21. Adjustment of the potential applied to terminal V onthe other hand, controls beam current while variation of the potentialapplied to terminal V adjusts the strength of the convergent lens.Finally adjustment of the potential applied to terminal V prevents lensaction of the corrected beam in its traverse to input coupler 12. Solong as the operating potentials of contiguous sections withincylindrical electrodes 25 and 30 are nearly alike, no appreciable lensaction occurs at their boundaries and, therefore, it is feasible to varythe operating potentials, within limits, to accomplish the severaladjustments described.

The specific dimensions and significant voltages applied to the electrongun in one embodiment, operated satisfactorily for an input signal at900 megacycles, were as follows:

Spacing of cathode to anode 20 inches .0232 Spacing of anode toprincipal plane of convergent lens inches .1675 Length of electrode 25do .1385 Minimum diameter of cylindrical electrodes inches .016 Diameterof anode aperture 21 do .010 Potentials applied to terminals V V V volts30 Potentials applied to terminals V and V volts Beam currentmicroamperes 66 Cathode current density ma./cm. 110 Electrode thicknessinches .005 Insulation thickness do .003

It is not necessary that electrodes 25 and 30 or their several sectionsbe circumferentially closed cylinders. Where thin, planar elementsconstituting the end plates of the cylinders or their several sectionshave a diameter that is large relative to the spacing of each plate fromits neighbor, a substantially field-free space may be established iftransverse plates constituting a given cylindrical electrode arestrapped so that the necessary pairs of plates are maintained at thesame operating potential. In other words, section 25a in the embodimentof FIGURE 2, for example, may be formed of a pair of parallel planarelectrodes, centrally apertured and bridged at spaced points aroundtheir peripheries so that they are maintained at a common potentialthrough the connection extending from terminal V In arriving at thedescribed structures, efiorts were first directed to establishing aconvergent lens immediately following anode 20 but it was found that thespacing of the lens in an axial direction to accomplish compensation wasso small that it could not be realized in a physical structure. Thespacing required for the compensating lens from anode 20 was calculatedto be less than the diameter of the anode aperture but, since anelectron lens has an axial length at least equal to the diameter of theelectrode apertures, such space requirements could not be satisfied.

This difficulty is overcome in the described structures where thecompensating lens is spaced sufliciently from anode 20 that the lenseffects are isolated from one another. A further advantage is realizedfrom this separation in that the perturbation experienced at the anodewould be stronger if the decelerating field of the compensating lenswere a factor in determining the divergence angle of the beam at anodeaperture 21. Since the perturbation developed on the beam at anodeaperture 21 is minimized in the described gun structures, the gun isless critical to adjustment of operating potentials. There is thefurther obvious advantage of a reduction in the number of electrodeswith an attendant simplification in the power supply. Parametricamplifiers to which the described electron guns have been adapted show amarked improvement in noise factor which is not, in any sense,critically dependent on operating potentials.

While particular embodiments of the invention have been shown anddescribed, modifications may be made, and it is intended in the appendedclaims to cover all such modifications as may fall within the truespirit and scope of the invention.

I claim:

1. An electrode system for producing and projecting along a path,extending coaxially of a homogeneous unidirectional magnetic field, anelectron beam having no substantial radial component of motioncomprising: a cathode; an anode having an aperture centered on saidpath; means for operating said anode at a positive potential withrespect to said cathode to draw electrons therefrom into a beam havingan initial radius determined by said aperture, said beam, as it emergesfrom said aperture, expands radially outwardly and this perturbationgives rise to a sinusoidal component of radial motion of a predeterminedscallop frequency; means, including a cylinder of nonmagnetic conductivematerial positioned coaxially of said path, for establishing about saidpath a substantially electric-field-free drift space for said perturbedbeam having a length approximately equal to an integral number of Wavelengths at said scallop frequency; and means for terminating said driftspace in a convergent lens which has a principal plane located at theintercept of said perturbed beam with an equilibrium radius larger thansaid initial radius and which subjects said perturbed beam to a radiallyinward force to cancel said component of radial motion.

2. An electrode system for producing and projecting along a path,extending coaxially of a homogeneous unidirectional magnetic field, anelectron beam having no substantial radial component of motioncomprising: a cathode; an anode having an aperture small in areacompared with said cathode and centered on said path; means foroperating said anode at a positive potential with respect to saidcathode to draw electrons therefrom into a beam having an initial radiusdetermined by said aperture, said beam, as it emerges from saidaperture, expands radially outwardly and this perturbation gives rise toa sinusoidal component of radial motion of a predetermined scallopfrequency; means, including a cylinder of non-magnetic conductivematerial positioned coaxially of said path, for establishing about saidpath a substantially electric-field-free drift space for said perturbedbeam having a length approximately equal to an integral number of wavelengths at said scallop frequency; and means for terminating said driftspace in a convergent lens which has a principal plane located at theintercept of said perturbed beam with an equilibrium radius larger thansaid initial radius and which subjects said perturbed beam to a radiallyinward force to cancel said component of radial motion.

3. An electrode system for producing and projecting along a path,extending coaxially of a homogeneous unidirectional magnetic field, anelectron beam having no substantial radial component of motioncomprising: a cathode; an anode having an aperture small in areacompared With said cathode and centered on said path; means foroperating said anode at a positive potential with respect to saidcathode to draw electrons therefrom into a beam having an initial radiusdetermined by said aperture, said beam, as it emerges from saidaperture, expands radially outwardly and this perturbation gives rise toa sinusoidal component of radial motion of a predetermined scallopfrequency; means, including a cylinder of non-magnetic conductivematerial positioned coaxially of said path, for establishing about saidpath a substantially electricfield-free drift space for said perturbedbeam having a length approximately equal to an integral number of wavelengths at said scallop frequency; and means for terminating said driftspace in a convergent lens which has a principal plane located at theintercept of said perturbed beam with an equilibrium radius larger thansaid initial radius and which subjects said perturbed beam to a radiallyinward force to cancel said component of radial motion, said last-namedmeans including a second cylinder of nonmagnetic conductive materialpositioned coaxially of said path adjacent said first-mentioned cylinderand providing a second substantially electric-field-free drift space forsaid beam.

4. An electrode system for producing and projecting along a path,extending coaxially of a homogeneous unidirectional magnetic field, anelectron beam having no substantial radial component of motioncomprising: a cathode; an anode having an aperture small in areacompared with said cathode and centered on said path; means foroperating said anode at a positive potential with respect to saidcathode to draw electrons therefrom into a beam having an initial radiusdetermined by said aperture, said beam, as it emerges from saidaperture, expands radially outwardly and this perturbation gives rise toa sinusoidal component of radial motion of a predetermined scallopfrequency; a first cylindrical electrode of non-magnetic conductivematerial positioned coaxially of said path, for establishing about saidpath a substantially electric-field-free drift space for said perturbedbeam having a length approximately equal to an integral number of wavelengths at said scallop frequency, said electrode having a centrallyapertured end plate at the end thereof closer to said cathode serving assaid anode; and a second cylindrical electrode of non-magneticconductive material positioned coaxially of said path adjacent saidfirst electrode at the end thereof remote from said cathode forestablishing about said path a second drift space for said beam, thecontiguous end portions of said first and second cylindrical electrodesconstituting a convergent lens which has a principal plane located atthe intercept of said perturbed beam with an equilibrium radius largerthan said initial radius and which subjects said perturbed beam to aradially inward force to cancel said component of radial motion.

5. An electrode system for producing and projecting along a path,extending coaxially of a homogeneous unidirectional magnetic field, anelectron beam having no substantial radial component of motioncomprising: a cathode; an anode having an aperture small in areacompared with said cathode and centered on said path; means foroperating said anode at a positive potential with respect to saidcathode to draw electrons therefrom into a beam having an initial radiusdetermined by said aperture, said beam, as it emerges from saidaperture, expands radially outwardly and this perturbation gives rise toa sinusoidal component of radial motion of a predetermined scallopfrequency; a first cylindrical electrode of non-magnetic conductivematerial positioned coaxially of said path, for establishing about saidpath a substantially electric-field-free drift space for said perturbedbeam having" a length approximately equal to an integral number of wavelengths at said scallop frequency, said electrode having a centrallyapertured end plate at the end thereof closer to said cathode serving assaid anode; a second cylindrical electrode of non-magnetic conductivematerial positioned coaxially of said path adjacent said first electrodeat the end thereof remote from said cathode for establishing about saidpath a second drift space for said beam, the contiguous end portions ofsaid first and second cylindrical electrodes constituting a convergentlens which has a principal plane located at the intercept of saidperturbed beam with an equilibrium radius larger than said initialradius and which subjects said perturbed beam to a radially inward forceto cancel said component of radial motion; and an input coupler fortransferring signal energy relative to said beam, positioned adjacentthe end of said second electrode remote from said cath ode, andmaintained at a direct-current potential substantially equal to that ofsaid second electrode.

6. An electrode system for producing and projecting along a path,extending coaxially of a homogeneous unidirectional magnetic field, anelectron beam having no substantial radial component of motioncomprising: a cathode; an anode having an aperture centered on saidpath; means for operating said anode at a positive potential withrespect to said cathode to draw electrons therefrom into a beam havingan initial radius determined by said aperture, said beam, as it emergesfrom said aperture, expands radially outwardly and this perturbationgives rise to a sinusoidal component of radial motion of a predeterminedscallop frequency; means, including a cylinder of non-magneticconductive material positioned coaxially of said path, for establishingabout said path a substantially electric-field-free drift space for saidperturbed beam having a length approximately equal to an integral numberof wave lengths at said scallop frequency; and means including a secondcylinder of non-magnetic conductive material positioned coaxially ofsaid path adjacent said first-mentioned cylinder, for terminating saiddrift space in a convergent lens which has a principal plane located atthe intercept of said perturbed beam with an equilibrium radius largerthan said initial radius and which subjects said perturbed beam to aradially inward force to cancel said component of radial motion, saidsecond cylinder comprising a plurality of mutually insulated sectionsand terminal connections for applying desired operating potentialsthereto.

7. An electrode system for producing and projecting along a path,extending coaxially of a homogeneous unidirectional magnetic field, anelectron beam having no substantial radial component of motioncomprising: a cathode; an anode having an aperture small in areacompared with said cathode and centered on said path; means foroperating said anode at a positive potential with respect to saidcathode to draw electrons therefrom into a beam having an initial radiusdetermined by said aperture, said beam, as it emerges from saidaperture, expands radially outwardly and this perturbation gives rise toa Sin soidal component of radial motion of a predetermined scallopfrequency; a first cylindrical electrode of nonmagnetic conductivematerial positioned coaxially of said path for establishing about saidpath a substantial electric-field-free drift space for said perturbedbeam having a length approximately equal to an integral number of wavelengths at said scallop frequency, said electrode having a centrallyapertured end plate at the end thereof closer to said cathode serving assaid anode; and a second cylindrical electrode of non-magneticconductive material positioned coaxially of said path adjacent saidfirst electrode at the end thereof remote from said cathode forestablishing about said path a second drift space for said beam, thecontiguous end portions of said first and second cylindrical electrodesconstituting a convergent lens which has a principal plane located atthe intercept of said perturbed beam with an equilibrium radius largerthan said initial radius and which subjects said perturbed beam to aradially inward force to cancel said component of radial motion, both ofsaid cylindrical electrodes comprising a plurality of mutually insulatedsections and terminal connections for applying desired operatingpotentials thereto.

References Cited by the Examiner UNITED STATES PATENTS 2,147,454 2/39Morton.

2,347,797 5/44 Posthumus et a1. 315-15 2,383,751 8/45 Spangenberg 314142,508,645 5/50 Linder.

2,817,035 12/57 Birdsall 3153.5 X 2,829,299 10/58 Beck 315-3.5 2,936,3945/60 Brewer 315--3.5 2,947,905 8/60 Pierce 315-35 X 2,972,702 2/ 61Kompfner et a1 315-3 X GEORGE N. WESTBY, Primary Examiner.

ARTHUR GAUSS, Examiner.

1. AN ELECTRODE SYSTEM FOR PRODUCING AND PROJECTING ALONG A PATH,EXTENDING COAXIALLY OF A HOMOGENEOUS UNIDIRECTIONAL MAGNETIC FIELD, ANELECTRON BEAM HAVING NO SUBSTANTIAL RADIAL COMPONENT OF MOTIONCOMPRISING: A CATHODE; AN ANODE HAVING AN APERTURE CENTERED ON SAIDPATH; MEANS FOR OPERATING SAID ANODE AT A POSITIVE POTENTIAL WITHRESPECT TO SAID CATHODE TO DRAW ELECTRONS THEREFROM INTO A BEAM HAVINGAN INITIAL RADIUS DETERMINED BY SAID APERTURE, SAID BEAM, AS IT EMERGESFROM SID APERTURE, EXPANDS RADIALLY OUTWARDLY AND THIS PERTURBATIONGIVES RISE TO A SINUSOIDAL COMPONENT OF RADIAL MOTION OF A PREDTERMINEDSCALLOP FREQUENCY; MEANS, INCLUDING A CYLINDER OF NONMAGNETIC CONDUCTIVEMATERIAL POSITIONED COAXIALLY OF SAID PATH, FOR ESTABLISHING ABOUT SAIDPATH A SUBSTANTIALLY ELECTRIC-FIELD-FREE DRIFT SPACE FOR SAID PERTURBEDBEAM HAVING A LENGTH APPROXIMATELY EQUAL TO TAN INTEGAL NUMBER OF WAVELENGTHS AT SAID SCALLOP FREQUENCY; AND MEANS FOR TERMINATING SAID DRIFTSPACE IN A CONVERGENT LENS WHICH HAS A PRINCIPAL PLANE LOCATED AT THEINTERCEPT OF SAID PERTURBED BEAM WITH AN EQUILIBRIUM RADIUS LARGER THANSAID INITIAL RADIUS AND WHICH SUBJECTS SAID PERTURBED BEAM TO A RADIALLYINWARD FORCE TO CANCEL SAID COMPONENT OF RADIAL MOTION.